The present invention relates to a highly practical speech recognition apparatus capable of accurately recognizing an input speech continuously produced.
An automatic recognition of a speech is considered to be very important as an interface technique capable of inputting information directly from a human being to a machine. In this automatic speech recognition, a speech pattern obtained by continuously generating a sequence of phonetic symbols such as phonemes, syllables, words is converted into discrete phonetic symbols, and these discrete phonetic symbols are recognized.
A speech is generated by the physical change of a human sound generating organ, and individual phonetic symbols are not discretely generated. In other words, a speech pattern of a certain phonetic symbol in a continuously produced speech is affected by the influence of the speech pattern of the phonetic symbols at the front and rear of the phonetic symbol. For instance, when a phoneme is used as a phonetic symbol, a speech pattern of a phoneme of [i] in a word of [eit] is affected by the influence of a speech pattern of phonemes [e] and [t] at the front and rear of the phoneme [i]. Accordingly, even if the speech pattern of the phonene [i] is picked up and reproduced, it is difficult to recognize the speech pattern as [i]. When the word is used as a phonetic symbol, the part [v] of [faiv] of a row of numeric characters "54" is frequently weakened, but in case of a row of numeric characters "51", they are not almost weakened. In other words, even the same numeric character 5 takes different speech pattern in response to the conditions of generating the speech.
A speech pattern of phonetic symbol of a speech continuously generated by a human being is frequently altered by the influence of the speech pattern at the front and rear of the speech pattern, as described above. Thus, it is almost impossible to represent a certain phonetic symbol in a single speech pattern, to ignore the influence of the speech pattern at the front and rear of the symbol and to correctly recognize the phonetic symbol. Therefore, it is heretofore considered to prepare as a reference pattern a plurality of speech patterns for a certain phonetic symbols. However, when a number of deformed reference patterns are necessary for the individual phonetic symbols or when there are many words to be recognized, total number of the reference patterns to be prepared increases to huge quantity, resulting in a large problem in a practical use.
It is also considered heretofore to recognize the speech pattern after the influence of the speech pattern is eliminated at the front and rear of a certain speech pattern. However, in this case, in order to obviate the influence of the front and rear speech patterns on the certain speech pattern, it is required to decide in advance the phonetic symbols represented by these front and rear speech patterns. In addition, in order to decide the phonetic symbols at the front and rear of the symbol, it is required to decide the phonetic symbol of the above-described certain speech pattern. Thus, the successive speech patterns affect each other, and it is not simple to remove the influence.